For years, it was an article of faith that if there was an important bipartisan deal to be struck on Capitol Hill, veteran Republican Sen. Olympia J. Snowe of Maine would be in the mix. Throughout struggles to find common ground on issues ranging from campaign finance reform and health care to economic stimulus and intelligence, the willowy, amiable Snowe was invariably a major player.

So her announcement on Tuesday that she was retiring after for more than three decades in Congress out of sheer frustration with the relentless, grinding partisanship on Capitol Hill was another sobering reminder that Congress has been rendered a political no-man’s land where compromise between the two armed camps is a rare exception.

“As I have long said, what motivates me is producing results for those who have entrusted me to be their voice and their champion,” Snowe, 65, said in a statement that caught her staff and much of Washington by surprise. “I do find it frustrating, however, that an atmosphere of polarization and ‘my way or the highway’ ideologies have become pervasive in campaigns and in our governing institutions.

Snowe, a gutsy politician who overcame overwhelming personal tragedy to carve out a career on Capitol Hill, said she had little doubt she could win a fourth term in the Senate if she wanted, despite Tea Party carping about her moderate voting record and strong bipartisan instincts. “However, what I have had to consider is how productive an additional term would be,” she added. “Unfortunately, I do not realistically expect the partisanship of recent years in the Senate to change over the short term.”

Indeed, the days of moderate Republicans and conservative Democrats with a propensity for compromise are sadly marked. The retirement at the end of this year of Democratic senators Kent Conrad of North Dakota and Ben Nelson of Nebraska, independent Joseph I. Lieberman of Connecticut and now Snowe continues a mass exit of lawmakers who occupied the political middle ground on Capitol Hill.

Other prominent Senate middle of the roaders, including Democrat Claire McCaskill of Missouri and freshman Republican Scott Brown of Massachusetts, are facing tough challenges from the opposite party, while veteran Republican Sen. Richard Lugar of Indiana is fighting to survive a primary challenge.

And it’s quite likely there will be a similar wave of moderate departures in the House: Reps. Dan Boren, D-Okla., and Mike Ross, D-Ark., are retiring; Rep. Joe Donnelly, D-Ind., is seeking a Senate seat; and Reps. Jim Matheson, D-Utah, Jason Altmire, D-Pa. and Mike McIntyre, D-N.C., all are facing tough reelection battles.

Just as Republican presidential frontrunner Mitt Romney has been battered from the right by social conservatives and Tea Party adherents, veteran Republican lawmakers who are perceived to be too accommodating or cozy with Democrats can find themselves in hot water back home.

Lugar, a conservative with a long history of bipartisanship in foreign policy and agriculture issues, is facing a Tea Party-backed challenge in the primary. Tea Party activists say they are disillusioned with Lugar because he has been willing to work across party lines on some issues, like the bank bailouts and gun control. He supports the DREAM Act, which would provide a pathway to citizenship for some illegal immigrants. And he voted to confirm President Obama’s nominations of Sonia Sotomayor and Elena Kagan to the Supreme Court. Tea Party Express leader Amy Kremer called Lugar "the epitome of what is wrong in Washington, D.C."

Former Republican senator Robert F. Bennett of Utah was ousted from office two years ago after Tea Party forces denied him nomination for another term. That lesson hasn’t been lost on Sen. Orrin G. Hatch of Utah, who is up for reelection this year. Hatch, a long-time dealmaker who once boasted of his strong ties to the late Sen. Edward M. Kennedy, D-Mass., is now one of the most dependable Republican votes in the Senate.

Washington Editor and D.C. Bureau Chief Eric Pianin is a veteran journalist who has covered the federal government, congressional budget and tax issues, and national politics. He spent over 25 years at The Washington Post.